This invention relates to the electrolytic manufacture of hydroxide solutions. More specifically, it relates to a process for producing alkali metal hydroxides in the form of concentrated aqueous solutions by the electrolysis of aqueous alkali metal halide solutions utilizing two different types of electrolytic cells. One type is a diaphragm cell, such as is presently commonly used in industry. The other type is a membrane cell which utilizes at least one permselective membrane.
Chlorine and caustic are essential and large volume chemicals which are required in all industrial societies. They are commercially produced by electrolysis of salt solutions, and a major portion of such production at the present time is by diaphragm cells.
Diaphragm cells useful in the production of chlorine and caustic are well known in the art. Typical of this type of cell is that designated as an "H-4" cell by Hooker Chemicals & Plastics Corp. This type of cell utilizes dimensionally stable anodes, which consist of an active surface of noble metals, alloys, or oxides, or mixtures thereof, deposited on a valve metal substrate. In the operation of a diaphragm cell, a nearly saturated alkali metal halide solution is fed into the anolyte compartment of the cell, from which it passes through a permeable diaphragm, usually of deposited asbestos, to the catholyte compartment, where under a decomposing current, alkali metal hydroxide is formed. Halide gas is formed at the anode, and hydrogen is formed along with alkali metal hydroxide at the cathode.
Membrane cells, or electrolytic cells utilizing permselective membranes to separate the anode and the cathode during electrolysis, are known in the art. For example, such cells are described in U.S. Pat. Nos. 3,899,403; 3,954,579; and 3,959,095. Within recent years, improved membranes have been introduced. The improved membranes are preferably utilized in the present invention. Such membranes are fabricated of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a sulfonated perfluorovinyl ether. More specifically, suitable membrane materials are fabricated of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether of the general formula: FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF.dbd.CF.sub.2, hereinafter referred to as PSEPVE. Generally, such polymers have an equivalent weight of from about 900 to about 1,600. In use, the membranes are usually supported on networks of supporting materials such as polytetrafluoroethylene, perfluorinated ethylene propylene polymer, polypropylene, asbestos, titanium, tantalum, niobium or noble metals. Utilizing an alkali metal halide feed, a membrane cell produces alkali metal hydroxide and hydrogen at the cathode and halide at the anode.
In processes utilizing membrane or diaphragm electrolytic cells, not all of the alkali metal chloride feed material entering the cell is electrolyzed; a part of the unreacted portion is withdrawn with the hydroxide solution product from the catholyte compartment. This mixture is generally referred to as "cell liquor." The cell liquor from diaphragm cells, for example, normally contains from about 9 to about 12 percent by weight alkali metal hydroxide, and 10 to 18 percent by weight alkali metal chloride together with some alkali metal sulfate. In order to obtain a commercial product, the alkali metal hydroxide must be concentrated and separated from the chlorides and sulfates which may be present. Separation and concentration of the alkali metal hydroxide product is usually accomplished by evaporation of the cell liquor. The evaporation process is normally carried out to produce a marketable product containing about 50 percent by weight alkali metal hydroxide, which product usually contains from about 0.8 to about 2.0 percent alkali metal chlorides.
The expense of evaporation of the alkali metal hydroxide to produce a marketable product that may economically be shipped over long distances is required when either diaphragm or membrane cells are utilized. The capital cost of electrolytic cells and related equipment has steadily increased over recent years. The expense of installing and operating evaporating equipment has also increased as has the cost of fuel or steam. The evaporation step has become the subject of much study toward effectuating more efficient and more economic processes. Rising capital costs have also curtailed expansions in chlor-alkali capacity, especially where a change from one type of electrolytic cell to another is a consideration. Although the present invention is not restricted thereto, it is particularly adapted to the economic expansion of capacity of an existing diaphragm cell plant.
In accord with the present invention, a method has been devised wherein two types of electrolytic cells, diaphragm and membrane cells, are operated in tandem to more economically produce alkali metal hydroxides. The present method produces a viable alternative to expansion of plant capacity. Expansion of installations to include both types of cells has an economic advantage over the expansion of either a wholly diaphragm or a membrane cell installation. The present invention produces a cell liquor of higher alkali metal hydroxide concentration than diaphragm cells normally produce. The concentrated cell liquor, in turn, requires less evaporation to produce a more desirable concentrated product, generally about 50 percent or more by weight alkali metal hydroxide. The present process also facilitates the addition of capacity to chlor-alkali plants to be accomplished by the addition of product capacity, that is, electrolytic cells which produce both chlorine and alkali metal hydroxides, without addition of processing capacity, e.g. evaporators, which process only alkali metal hydroxide solutions.